Ethanol can be produced from grain-based feedstocks (e.g., corn, sorghum/milo, barley, wheat, soybeans, etc.), from sugar (e.g., from sugar cane, sugar beets, etc.), and from biomass (e.g., from lignocellulosic feedstocks such as switchgrass, corn cobs and stover, wood or other plant material).
Biomass comprises plant matter that can be suitable for direct use as a fuel/energy source or as a feedstock for processing into another bioproduct (e.g., a biofuel such as cellulosic ethanol) produced at a biorefinery such as an ethanol plant. Biomass may comprise, for example, corn cobs and stover (e.g., stalks and leaves) made available during or after harvesting of the corn kernels, fiber from the corn kernel, switchgrass, farm or agricultural residue, wood chips or other wood waste, and other plant matter. In order to be used or processed, biomass will be harvested and collected from the field and transported to the location where it is to be used or processed.
In a conventional ethanol plant producing ethanol from corn, ethanol is produced from starch. Corn kernels are cleaned and milled to prepare starch-containing material for processing. Corn kernels may also be fractionated to separate the starch-containing material (e.g., endosperm) from other matter (e.g., fiber and germ). The starch-containing material is slurried with water and is liquefied to facilitate saccharification where the starch is converted into sugar (e.g., glucose) and fermentation where the sugar is converted by an ethanologen (e.g., yeast) into ethanol. The product of fermentation is beer, which comprises a liquid component containing ethanol, water, and soluble components, and a solids component containing unfermented particulate matter among other things. The fermentation product is sent to a distillation system where it is distilled and dehydrated to yield ethanol. The residual matter (e.g., whole stillage) comprises water, soluble components, oil and unfermented solids (i.e., the solids component of the beer with substantially all ethanol removed that can be dried into dried distillers grains (DDG) and sold as an animal feed product). Other co-products, for example syrup (and oil contained in the syrup), can also be recovered from the stillage. Water removed from the fermentation product in distillation can be treated for re-use at the plant.
In a biorefinery configured to produce ethanol from biomass, ethanol is produced from lignocellulosic material. Lignocellulosic biomass typically comprises cellulose, hemicellulose, and lignin. Cellulose (a type of glucan) is a polysaccharide comprising hexose (C6) sugar monomers such as glucose linked in linear chains. Hemicellulose is a branched chain polysaccharide that may comprise several different pentose (C5) sugar monomers (e.g., xylose and arabinose) and small amounts of hexose (C6) sugar monomers (e.g., mannose, galactose, rhamnose and glucose) in branched chains.
In a typical cellulosic process, the biomass is prepared so that sugars in the lignocellulosic material (e.g., glucose from the cellulose, and xylose from the hemicellulose) can be made accessible and fermented into a fermentation product containing ethanol. After fermentation, the fermentation product is distilled and dehydrated to yield ethanol.
In the preparation of the biomass for fermentation, the biomass is typically pretreated, for example, using an acid such as sulfuric acid. In order to achieve high ethanol concentration from the fermentation of acid-pretreated biomass (e.g., corn cobs), the C6 sugar-containing stream of the pretreated biomass is ideally fed into an enzyme hydrolysis reaction (i.e., a saccharification reaction) at a high solids loading. However, mixtures of acid-pretreated biomass (e.g., corn cobs) above about 10% solids are typically viscous and difficult to process in a traditional stirred tank reactor. As a result, it is typical for the enzymatic hydrolysis reaction to be carried out in either a fed batch mode or at a low solid loading. This lowers the efficiency of the process, however, and results in a lower concentration (i.e., a lower titer) of ethanol in the resulting fermentation product.
In view of the above, it would be advantageous to provide a system that provides one or more features to facilitate improvement in the efficiency and yield of ethanol from biomass.